Sex = Sum total of those differences in structure and function on the basis of which an organism is classified as male or female. Theories of sex determination:  Chromosomal theory  Genic balance theory  Genetic dosage compensation / X-inactivation model of sex determination  Temperature dependent sex determination in alligators

The chromosomal theory of sex determination  According to this theory, sex is determined at fertilization by the sex chromosomes.  Each gamete contains the haploid number of chromosomes.  In mammals and in most insects, all gametes produced by the female are similar, having one X chromosome.  Males on the other hand produce two types of gametes in approximately equal numbers, one type bearing the X chromosome and the other type bearing the Y chromosome.  Hence in mammals and most insects, the female is known as the homogametic sex while the male is heterogametic.

The chromosomal theory of sex determination, contd.  At fertilization, the union of an egg with an X-carrying spermatozoon results in a zygote with 2 X chromosomes which develops into a female while the union of a Y-carrying spermatozoon with an egg results in an individual with one X and one Y chromosome, which develops into a male.  In birds, the female is the heterogametic sex while the male is homogametic (Table 2.1).  Chromosomal, genotypic or genetic sex is thus determined by the sex chromosomes received from the parents.

The chromosomal theory of sex determination, contd.  Aberrations of genetic sex do occur, either as a result of non- disjunction of sex chromosomes, translocation, deletion or mutation.  In maternal non-disjunction, fertilization of the ovum will lead to formation of either an XXX or an XXY zygote. In paternal non- disjunction, zygotes of XXY or XO genotypes result. Such cases of aneuploidy often result in gonadal or endocrine defects.  In man, the Klinefelter's syndrome in which the genotype is usually XXY instead of XY is characterized by gonadal hypofunction.  In women showing Turner's syndrome which is characterized by gonadal agenesis or aplasia, the genotype is frequently XO rather than XX.

Table 2.1 Kinds of chromosomal sex determination

Genic balance theory of sex determination  To some extent, genotypic sex may be considered an "all or none" or qualitative trait since usually male or female zygotes are formed at fertilization.  However, sex is a phenotypic trait, determined by interaction between genotype and environment. Individuals vary in their degrees of maleness or femaleness.  C.B. Bridges in 1922 proposed the genic balance theory to explain apparent quantitative variability in sexual character.

Genic balance theory of sex determination, contd.  According to this theory, sex is determined by the autosomes as well as by the X chromosomes, the ratio of autosomes to X's being the significant relation.  In Drosophila the X chromosome carries more genes for femaleness while the autosomes carry more genes for maleness (Table 2.2).  Which sex actually develops is decided by the balance between the two sets of genes.

Table 2.2 Sexual types in the fruit fly, Drosophila melanogaster

Genetic dosage compensation / X-inactivation model of sex determination  The theory of mammalian X-chromosome inactivation proposed by Lyon in 1961 holds that almost all the genes on one of the two X-chromosomes in the somatic cells of females are suppressed as a dosage compensation mechanism.  The choice of either the paternal or maternal X chromosome for inactivation is random.  The inactivated X chromosome is identifiable as it is heterochromatic and usually has its DNA replicated later in mitosis than other chromosomes.  The ZFY gene, thought to constitute the primary sex-determining signal, was identified within a very small segment of the Y chromosome by Page and co-workers in  However, the presence of a similar gene, ZFX, on the X chromosome prompted the propounding of the dosage compensation / X-inactivation theory of sex determination.

Genetic dosage compensation / X-inactivation model of sex determination, contd.  According to this theory, both ZFX and ZFY produced functionally interchangeable proteins.  It therefore follows that XY cells would have two active copies of the gene while XX cells would only have one active copy, due to X-inactivation.  Embryos with two copies would thus develop into males while those with a single copy of the gene would develop into females.  In this way, the theory held that gene dosage determined sex.  Though elegant, the theory became untenable when it later became evident that the ZFX gene escapes X- inactivation, thus contradicting the dosage compensation model.  According to this theory, both ZFX and ZFY produced functionally interchangeable proteins. It therefore follows that XY cells would have two active copies of the gene while XX cells would only have one active copy, due to X- inactivation. Embryos with two copies would thus develop into males while those with a single copy of the gene would develop into females.  In this way, the theory held that gene dosage determined sex.  Though elegant, the theory became untenable when it later became evident that the ZFX gene escapes X- inactivation, thus contradicting the dosage compensation model.

Sex determination in birds  In the avian species, unlike the case in mammals, the female is the heterogametic sex.  In order to distinguish animals with female heterogamety from those with male heterogamety, the distinguishing chromosome is known as W in the former (in mammals, Y), while the common chromosome is called Z (X in mammals).  In mammals, the presence of the Y chromosome provides the primary genetic switch towards the formation of the testis which leads to development along the male line.  The SRY gene located within the sex determining region of the Y chromosome is the sex determining factor and is present on all mammalian Y chromosomes.  By contrast, no gene similar to the SRY in mammals has been found uniquely located on the avian W chromosome.  The sex determining mechanisms in birds and mammals therefore appear to be different.

Sex determination in birds, contd.  In birds, the genetic switch between male and female development is not known.  However, the evidence mostly supports a genic balance model as elucidated by C.B. Bridges for the fruit-fly, Drosophila melanogaster.  It appears that the sex of birds is determined by the ratio of Z chromosomes to autosomal chromosomes.  Results from the screening of commercial broiler chicken flocks indicate the presence of ZO females and ZZW males, consistent with the genic balance theory of sex determination in birds.

Temperature dependent sex determination in alligators  Many reptiles, including alligators and crocodiles, exhibit no sex chromosome dimorphism.  Rather, the sex of the offspring is determined by the temperature at which the eggs are incubated. In alligators (Alligator mississippiensis), incubation of eggs from: o C result in 100% female offspring, o C result in 100% male offspring, 32 o C o C different sex ratios, 35 o C - 100% females. Below 29 o C and above 35 o C, high mortality result.